Many industrial process control systems use control valves to control the flow rates of process fluids through pipes. Conventionally, a control valve may be opened and closed via an actuator, wherein a position of the actuator may be set according to positioning signals generated via a positioner based upon process settings, feedback from an actuator position sensor, and/or other input. In some process environments, the actuator may be a double-acting pneumatic actuator comprising two pneumatic chambers, wherein a pressure differential of pneumatic fluid supplied to each chamber pushes bidirectionally on a piston connected to a stem. The stem, in turn, translates the motion of the piston to adjust an opening of the control valve to increase or decreased flow rate of a process fluid.
A primary goal of a valve positioner and actuator combination is to quickly and accurately control fluid flow through the control valve via controlling actuator position. Another major goal of the combination is to minimize undesirable deviations in the actuator position as a result of forces generated by the flow of the process fluid itself, and/or other environmental forces.
Susceptibility of the actuator position to such forces may vary based upon a stiffness of the actuator, which, in the case of the double-acting pneumatic actuator, may be expressed as an average of the pressures (this average sometimes referred to as “crossover pressure”) of the two pneumatic chambers. Low actuator stiffness (i.e., low crossover pressure between the two chambers) may leave the actuator more susceptible to position deviations caused by process fluid forces and/or other forces, which may additionally cause increased wear and tear on the actuator. A high actuator stiffness (i.e., high crossover pressure), may impede desired actuator movement.
Thus, actuator stiffness presents an engineering trade-off between ability to rapidly control actuator position and mitigation of buffeting forces that may cause undesired deviations in the actuator position. Conventionally, stiffness in a double-acting pneumatic actuator may be adjusted via a mechanical adjustment, often in the form of a pneumatic bleed to “drain” undesired pressure from the pneumatic chambers. However, such practices may be time-consuming, inexact, and wasteful of pneumatic fluid. Ability to more precisely define and maintain actuator stiffness would therefore improve quality, durability, and efficiency in a process control environment.